Electronic devices such as personal computers and video devices have incorporated therein an electronic circuit which operates in the region of high frequency waves. Solid electrolytic capacitors are excellent in high-frequency characteristics, small-sized and great in capacity and have therefore found wide use in such electronic circuits. Most common solid electrolytic capacitors include a solid electrolytic capacitor 1 of the chip type shown in FIG. 5. The capacitor 1 comprises a capacitor element 2 having the function of a capacitor. The capacitor element 2 is fabricated with a block of anode body 3 serving as a base. Useful as the anode body 3 is a sintered body of a valve metal such as tantalum, niobium, titanium or aluminum. A rodlike anode lead member 4 is implanted in the anode body 3 and has a portion projecting outward from one side of the anode body 3. For example, a wire of tantalum is used as the anode lead member 4.
A very thin dielectric coating 5 is formed over the surface of the anode body 3. The dielectric coating 5 is formed by oxidizing the surface of the anode body 3, for example, by anodic oxidation. An electrically conductive high polymer layer 7 is formed on the dielectric coating 5 by chemical polymerization or electrolytic polymerization. For example, polypyrrole is used for the conductive high polymer layer 7. Further formed over the high polymer layer 7 are a carbon layer 8 and a silver layer 9. An anode terminal 10 in the form of a plate is joined to the anode lead member 4, and a platelike cathode terminal 11 to the silver layer 9. The capacitor element 2 is covered with a packaging resin 12 in a rectangular form. For example, an epoxy resin is used as the packaging resin 12. The anode terminal 10 and the cathode terminal 11 are caused to extend from the packaging resin 12 in opposite directions and bent downward. The outer ends of these terminals 10, 11 are arranged along the bottom surface of the packaging resin 12 and used for soldering the capacitor 1 to a board (not shown) for mounting thereon.
As previously described, the conductive high polymer layer 7 is formed by chemical polymerization or electrolytic polymerization in the process for fabricating the capacitor element 2. For chemical polymerization, a monomer is subjected to oxidation polymerization using an oxidizer to form a conductive high polymer layer 7. Stated more specifically, after the dielectric coating 5 is formed on the anode body 3, an oxidizer is applied to the coating 5 for chemical polymerization. The anode body 3 having the oxidizer applied thereto is immersed in a solution of a monomer, or allowed to stand in an atmosphere of the monomer, whereby the monomer is polymerized on the dielectric coating 5 to form a conductive high polymer layer 7.
The chemical polymerization process has the drawback that the conductive high polymer layer 7 formed is low in strength and uneven in thickness. Presently, the electrolytic polymerization process is in wide use for forming the conductive high polymer layer 7. FIGS. 6A and 6B are diagrams for illustrating a procedure for fabricating a capacitor element 2 using the conventional common electrolytic polymerization process. FIG. 6A shows an anode body 3 having a precoat layer 6 made of an electrically conductive high polymer and covering a dielectric coating 5. The precoat layer 6 is formed by the chemical polymerization process.
After the precoat layer 6 has been formed, an electrically conductive high polymer layer is formed by the electrolytic polymerization process utilizing the precoat layer 6. As shown in FIG. 6B, the anode body 3 is immersed in a solution of the monomer to be polymerized. A vessel containing the solution is provided with an electrode plate 20. When voltage is applied across the precoat layer 6 serving as a positive electrode and the electrode plate 20 serving as a negative electrode, the monomer is subjected to oxidation polymerization to form an electrically conductive high polymer layer over the precoat layer 6.
With the conventional electrolytic polymerization process, the precoat layer 6 is given a positive potential by holding the end of an electrode member 21, which is electrically connected to the positive electrode of a power source, in contact with the precoat layer 6 as shown in FIG. 6B. However, the amount of current flowing through the precoat layer 6 varies with the degree of contact of the electrode member 21 with the precoat layer, so that extreme difficulty is encountered in giving a specified thickness to the conductive high polymer layers of the individual capacitor elements to be fabricated. Furthermore, if the electrode member 21 is moved after the conductive high polymer layer has been formed, a portion of the conductive high polymer layer formed on the anode body 3 frequently adheres to the end of the electrode member 21, as separated from the anode body 3.
JP-A No. 11-121280 discloses a method of solving the above problem. With this method, the precoat layer 6 covering the dielectric coating 5 is so formed as to cover also a projecting portion of the anode lead member 4. Since the precoat layer 6 is electrically connected to the anode lead member 4, a positive potential can be given to the precoat layer 6 by supplying electricity to the anode lead member 4 as shown in FIG. 7B. With this method, a portion of the precoat layer 6 and a portion of the conductive high polymer layer are made into an insulator to electrically separate the anode lead member 4 from the precoat layer 6 and the conductive high polymer layer after the high polymer layer has been formed.
This method nevertheless forms a burr 70 of conductive. high polymer around the anode lead member 4 as shown in FIG. 7C. The burr 70 grows along the liquid level of the solution during electrolytic polymerization and therefore extends in the form of a flange around the lead member 4. When the anode body 3 is immersed in the solution for electrolytic polymerization, a portion of the solution ascends along the anode lead member 4 from the liquid level due to surface tension. The burr 70 includes a portion formed along the lead member 4 owning to the ascent of the solution.
The burr 70 is of course unnecessary and accordingly a deburring step needs to be performed using a file, grinder or the like after the conductive high polymer layer 7 has been formed. However, the shape and size of the burr 70 differ from anode body to anode body. For this reason, the method described encounters the problem that the grinder or the like for removing the burr 70 is difficult and cumbersome to position in place. Another problem also arises in that even a portion which need not be ground is so treated in order to ensure reliable deburring. For example, if the conductive high polymer layer 7 is ground, the capacitor element permits an increased leakage current to impair the performance of the solid electrolytic capacitor.
The present invention, which has been accomplished to solve the foregoing problems, provides a process for fabricating a capacitor element wherein a conductive high polymer layer is formed by electrolytic polymerization by supplying current through an anode lead member, and the anode lead member can thereafter be deburred easily. The invention further provides a process for fabricating a capacitor element wherein a conductive high polymer layer can be formed by electrolytic polymerization by supplying current through an anode lead member without permitting formation of burrs.